1. Technical Field
The present invention relates to an abnormality detection device and an abnormality detection method for detecting abnormality of a reducing agent injection valve.
The present invention particularly relates to an abnormality detection device and an abnormality detection method for detecting predetermined clogging phenomena occurring in a reducing agent injection valve that injects a reducing agent for purifying nitrogen oxides in exhaust gas into the inside of an exhaust pipe.
2. Related Art
Conventionally, nitrogen oxides (NOx) are included in exhaust gas exhausted from internal combustion engines of automobiles and the like. As one exhaust gas purification system that reduces and purifies the NOx, there is an exhaust gas purification system configured such that a NOx purification catalyst is disposed in an exhaust pipe of an internal combustion engine and a reducing agent such as an aqueous solution of urea or unburned fuel is injected on the upstream side of the NOx purification catalyst.
This exhaust gas purification system is configured to promote, in the NOx purification catalyst, a reduction reaction between the reducing agent and the NOx in the exhaust gas to thereby break down the NOx into nitrogen, water, and carbon dioxide and release the nitrogen, water, and carbon dioxide into the atmosphere.
As one aspect of a reducing agent injection device with which this exhaust gas purification system is equipped, there is an injection-type reducing agent injection device that supplies the reducing agent directly to the inside of the exhaust pipe via a reducing agent injection valve.
Here, when an aqueous solution of urea is used as the reducing agent, the aqueous solution of urea has the property that it freezes and crystallizes in a predetermined temperature range, and it is easy for the aqueous solution of urea to cause clogging of the reducing agent injection valve.
Further, sometimes, regardless of the type of reducing agent, clogging of the reducing agent injection valve occurs as a result of particulate matter of soot or the like or unburned fuel included in the exhaust gas entering the inside of the reducing agent injection valve and carbonizing. When clogging occurs in the reducing agent injection valve, it becomes difficult to inject a target quantity of the reducing agent into the inside of the exhaust pipe, the quantity of the reducing agent that should be supplied to the NOx purification catalyst becomes insufficient, and exhaust gas purification efficiency ends up deteriorating.
Thus, as a method of detecting clogging of the reducing agent injection valve, there has been disclosed a method that uses a pressure drop quantity inside a supply path interconnecting the reducing agent injection valve and a pump to determine whether or not there is clogging (see JP-A-2008-180193).
More specifically, this method determines, in an injection-type injection device, clogging of the reducing agent injection valve by comparing a pressure drop quantity at a time when the valve is open, which represents a pressure drop quantity inside the supply path whose pressure has dropped in a predetermined amount of time when pressure-feeding by the pump has been stopped in a state where the reducing agent injection valve is in an injection mode, and a pressure drop quantity at a time when the valve is closed, which represents a pressure drop quantity inside the supply path whose pressure has dropped in a predetermined amount of time when pressure-feeding by the pump has been stopped in a state where the reducing agent injection valve is in a completely closed mode.
This determination method is configured to determine that there is clogging in the reducing agent injection valve when the difference between the pressure drop quantity at the time when the valve is open and the pressure drop quantity at the time when the valve is closed has become equal to or less than a predetermined value.
Further, as a method that uses a pressure drop quantity inside the supply path to determine whether or not there is clogging, there has also been disclosed the method described below (see JP-A-2008-202469).
More specifically, for example, this reducing agent path clogging determination method includes the steps of: controlling the drive duty of the pump such that a value detected by a pressure sensor disposed in a first reducing agent path (a path disposed between the pump and the reducing agent injection valve) is maintained at a predetermined value to thereby discriminate whether or not the drive duty of the pump is less than a predetermined threshold value; stopping the pump and completely opening the reducing agent injection valve when it has been discriminated that the drive duty of the pump is less than the predetermined threshold value to thereby calculate a pressure drop quantity where the pressure value inside the first reducing agent path detected by the pressure sensor drops in a predetermined amount of time; and discriminating whether or not clogging is occurring in the first reducing agent path or a second reducing agent path (a path that is connected to the reducing agent injection valve and is for allowing the reducing agent to circulate) on the basis of the pressure drop quantity that has been calculated.
Incidentally, there are cases where clogging in which all or most of the injection hole in the reducing agent injection valve is blocked (hereinafter called “complete clogging”) occurs and cases where clogging that is not complete clogging but in which part of the injection hole is blocked (hereinafter called “partial clogging”) occurs.
In partial clogging, the extent of clogging differs depending on the degree of blockage of the injection hole. Additionally, when the extent of clogging differs, a difference will arise in the quantity of reducing agent that is injected from the reducing agent injection valve in a predetermined open valve time period.
Here, the clogging determination methods described in JP-A-2008-180193 and JP-A-2008-202469 are configured to compare a pressure drop quantity with a predetermined threshold value.
That is, the clogging determination methods of JP-A-2008-180193 and JP-A-2008-202469 can obtain a rough determination effect of determining whether or not clogging is occurring, but they cannot obtain a fine determination effect of determining the extent to which clogging is occurring.
Consequently, when the extent of clogging cannot be accurately determined, it becomes difficult to accurately perform correction of the injection quantity of the reducing agent due to clogging, and, as a result, the deterioration in exhaust gas purification efficiency until clogging is eliminated becomes unable to be kept to a minimum.
Further, when the extent of clogging cannot be determined, it becomes difficult to judge what action should be taken thereafter in response to the extent of clogging, such as whether the reducing agent injection valve should be replaced or whether the clogging can be addressed with injection quantity correction.
Thus, the inventors of the present invention made every effort to discover that this problem can be solved by disposing plural threshold values for performing determination of clogging of the reducing agent injection valve and comparing these threshold values with the pressure inside the reducing agent supply path, and thus the inventors completed the present invention.
That is, it is an object of the present invention to provide a reducing agent injection valve abnormality detection device and abnormality detection method that can detect the extent of clogging of a reducing agent injection valve.